A couple of weeks ago, ground teams at NASA’s Kennedy Space Center in Florida removed one of the four main engines from the Space Launch System rocket slated to send four astronauts on a voyage around the Moon next year.

NASA officials ordered the removal of one of the massive rocket’s RS-25 main engines after discovering a hydraulic leak on the engine’s main oxidizer valve actuator, which controls the flow of super-cold liquid oxygen propellant into the engine’s main combustion chamber, an agency spokesperson told Ars.

This is the first time NASA has replaced a main engine on the SLS core stage. The space agency earmarked individual RS-25 engines for the first four flights of the Space Launch System rocket a decade ago. Those assignments were locked in until now.

Artemis II is scheduled for launch in early 2026. The schedule is primarily driven by the readiness of the Orion spacecraft. “The replacement does not affect the mission timeline, and the engine with the leak will be repaired for a future flight,” the NASA spokesperson said.